Appropriate direct-conversion X-ray detectors can be used in the field of X-ray imaging, e.g. computed tomography, angiography or radiography. The X-ray radiation or the photons can be converted into electrical pulses by a suitable sensor. Examples of sensor materials include CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs, etc. The electrical pulses are analyzed by evaluation electronics, e.g. an integrated circuit (Application Specific Integrated Circuit: ASIC) in a substrate.
In appropriate X-ray detectors having direct-conversion semiconductor sensors, electron hole pairs are generated by absorption of X-ray quanta in the sensor. The electron hole pairs are separated by an electric field which is applied to the sensor. The charges induce a charge pulse in the electrodes on both sides of the sensor. In the detector elements of the X-ray detector, the charge pulse is carried from a read-out contact to an input of the signal processing chain of the detector element. A first component of this chain is typically a pre-amplifier, e.g. a charge-sensitive pre-amplifier or a transimpedance amplifier. The input capacitance of the pre-amplifier has an influence on its noise characteristic. The input capacitance of the pre-amplifier also has an influence on the design of the pre-amplifier in respect of power consumption and transfer function.
Standard read-out contacts are typically used to connect the sensor to the inputs of the integrated circuit (ASIC) using soldered connections, e.g. solder balls. In this case, for reasons of stability, via connections are often used to connect some or all metal layers below the read-out contact. A via connection is a through-hole. The via connection represents an electrically conductive connection. The via connection can provide an electrically conductive connection between two metal layers or metallization layers. The diameter of the read-out contact is determined by the size of the solder balls to be used as a soldered connection to the sensor. The solder balls are selected to be as large as possible in order to maximize the distance between sensor and top metal layer and therefore to minimize the capacitance between the read-out contact and the integrated circuit.
In order to reduce the capacitance of the via connections below the read-out contact relative to proximate conducting paths in the integrated circuit, a metallization-free zone can be configured around the via connection. Both measures result in a high demand for space for the read-out contact and the structures below it, e.g. via connection and metallization-free zone. In the case of highly integrated pixel electronics with small distances between the individual detector elements, these measures represent a problem. Furthermore, light which has penetrated the sensor can reach into the integrated circuit as a result of the metallization-free zone and influence the analog electronics, e.g. the pre-amplifier. For example, the light can generate charge carriers in the sensitive analog circuits. The response of the sensitive analog amplifier circuits can be changed and degraded thereby.